East of the trainer is a mining cart that will take you to another area of Clay Tunnel, but don't go in there just yet. First, walk North West. A hidden Blue Shard can be found in a rock near the first left turn.

Further North is a small set of stairs down to a Full Heal. You won't be able to reach the other side from here unless you have a Pokemon with Rock Smash.

The cart will drop you off at the North East part of area two. A bit South is a boulder which can be pushed West. This will save you the trip of going all the way around the water, but you will miss out on a few items and trainers.

South is a Red Shard in a corner. Further South is the third trainer of the cave.

The mining cart takes you to the Northern part of Area one. Push the boulder to the South to open up the path back to Driftveil City. If your team is injured, you can heal up back there. South of the boulder is a Rare Bone at a dead end.

Next, push the boulder at the North East part of the cave. Push it East into a hole to open up the path to another mine cart. Before using the cart, pick up the Green Shard below. Take the mine cart to a small room where another mine cart is located, then take the new cart back to area three.

From this area, surf North across the water to a door. Before going in the door, investigate the rock near it to find a hidden Iron.

The door leads back to the small room with the mine carts. Push the boulder East into the hole, then take the third cart to the North East part of Area three.

Back in Area three, push the boulder West to open the area up. Go North to the next door, but investigate the rock near the door first. The rock contains a hidden Big Nugget. Now enter the door to a big room and a Mysterious Door.

The Underground Ruins hold the Mystery Door. It is a slightly hidden area in Clay Tunnel.

Through the door is nothing but rocks and a few ruins with inscriptions. To change the inside, return to the front of the door where the circle is. From the circle, take six steps down and nine steps to the right, then press A. A hidden switch is located here. Say yes to change the inside of the ruins.

Regirock was originally one of three Regi Pokemon. Last generation, a fourth Regi was added. Only two Regis can be caught in each game, but if you capture the third by using the Key System, you'll gain access to the fourth.

After capturing Regirock, you'll obtain a Key that alters the ruins for another Regi Pokemon. The next regi depends on which version you're playing. If it's Black 2, you'll find Registeel. If it's White 2, you'll find Regice. If you want to capture him now, you can.

Exit out of the ruins and save your game again. Return to the main menu of your game (Hold L + R + START = SELECT). Go to Unova Link > Key System > Key Settings. The last setting is the Mystery Door. Change to the newly unlocked chamber to alter the ruins for the new Regi. Start the game back up and enter the Mystery Door to see the new chamber. Make sure to save before capturing, again.